Distribution and evolution of organic matter phases during biochar formation and their importance in carbon loss and pore structure

This study investigated the distribution and evolution of organic phases during biochar formation from twelve waste biomass and at the highest heating temperatures between 200°C and 650°C. Relation of the organic phase transformation to the carbon loss and pore structure was also analyzed. The organic phases in both feedstock biomass and the derived biochar were sequentially separated into four fractions: neutral detergent soluble fraction, hemicellulose, cellulose, and lignin. Plant-based residues mainly contained cellulose (25.8-64.6%), while municipal solid wastes had a large fraction of neutral detergent soluble fraction (22.9-65.0%). Transformation of cellulose into detergent soluble fraction and hemicellulose initially happened at the charring temperature between 100°C and 200°C, and the complete transformation was observed at the higher temperatures from 200°C to 350°C. The high lignin amount in biochar may be partly formed from the aromatization of cellulose fraction in addition to the contribution from the existing lignin in feedstock. All biochars had small total pore volumes ranging from 0.009cm³g^-1 to 0.278cm³g^-1 and were a type of mesoporous material with the pore sizes between 2nm and 30nm. The decrease of detergent soluble fraction, hemicellulose, and cellulose fractions had little effect on pore formation, but it was positively related to the C loss during pyrolysis. Pore volume and surface area seemed to have a positive relationship with lignin percentage and insoluble minerals in feedstock. This study provided insight into the mechanism of biochar formation related to the C loss and pore structure evolution. It will help produce the designated biochar with different environmental functions.